If they didn’t, why would human sperm contain “killer sperm” that kills competing sperm in the vagina & elsewhere? Why would we need this if pre-historic human females weren’t getting impregnated from several different males on a regular basis?
I don’t know anything about “killer sperm”, except that I am tempted to add “Dude!” to the end of the phrase. However, you may have noticed that humans are not biologically monogamous, as are geese and a few other animals- wolves, maybe? Since we don’t have a heat cycle as such, the more sex, the better chance of pregnancy, and the better the species survives. The concept of monogamy wasn’t popular until people were advanced enough to figure out how babies were made, and that if you wanted your children to inherit your property, you had to make sure they were your kids.
To sum up- Nookie Good.
A quick Google search doesn’t support the existence of “killer sperm”: cites include this link and this link from the first page of results. The first result for “killer sperm” that supports the idea provides no evidence and seems to be someone’s personal web site.
That said, my understanding is that “in olden days” it was not uncommon for a woman to have multiple partners while trying to become pregnant: if the paternity was uncertain, the theory was that all of the potential fathers would help protect/provide for the baby. It probably didn’t work perfectly, but I’m sure several babies had more than one man looking out for them back when such duties were left to men.
In any case, I’m inclined to think that the concept of inheritances came about well after people figured out how babies were made. 
No killer sperm! But how will my boys ace out the competition? 
Swim! Swim for your lives!!
What are you asking, why semen is a spermicide or why isn’t semen a spermicide?
I don’t know where the study is right now but I do remember reading once that the penis acts as a pump in the vagina, pulling other people’s semen out so the males semen has less competetion. Also people tend to have sex after a fight, or after they have been away for a long time (ie when the woman is most likely to stray and have sex with someone else). So that is also a factor in males sexually competeting to impregnate the same woman.
Here you go
http://news.bbc.co.uk/2/hi/health/3128753.stm
The team from the State University of New York believe the thrust of the penis during sex may help to clear a woman’s reproductive system of a previous lover’s semen.
They tested their theory in experiments using latex phalluses, an artificial vagina and a mixture of starch and water.
New Scientist magazine reports they found the coronal ridge of the penis, found where the glans, or head, meets the shaft, could scoop out more than 90% of the cornstarch mixture with just one thrust…They also say they have evidence, from surveys of students, to indicate that sex tends to be more energetic if the women is suspected of cheating, or if the couple has been apart - suggesting a sub-conscious desire on the part of the man to rid his partner of any trace of another’s semen.
If chimps are any indication, our early ancestors were probably very promiscuous. Chimps and bonobos have sex frequently with many different partners (as well as same-sex partners.) Early man was probably similar. It was most likely only after our society grew more complex that the concepts of monogomy and jealousy came into play.
In the chimp world, sex is part of the glue that holds their societies together. As others mentioned, it’s a way of “making up” after conflicts, as well as cementing bonds, showing deference, and just plain having fun.
Remember that our ancestors may not have necessarily understood that sex makes babies, especially if they were “gettin’ it on” very frequently. Children probably were cared for by the females in a sort of communal fashion-- as some apes care for their babies. The males probably “supported” their favorite females by gifts of food which were rewarded with sex.
Hmmm… the more things change…
So let’s get this straight…
“Scientists” got a dildo, and a cornstarch-filled fleshlight…and then pumped them together. Am I the only one who finds this amusing?
LOL, can you imagine explaining how your day was at an innocent gathering that evening, like meeting spouse and a bunch of their friends and workmates in a bar for drinks…
So, what do you do…
I used a dildo and fleshlight to determine if the penis can remove a previous mans sperm from a vagina
:eek:
Could be worse, what do the grandmothers and mothers of people who actually design marital aids say their little precious does for a living 
[continued hijack]
You should rent a few episodes of Showtime’s Family Business: it’s about Adam Glasser, known in the porn industry as Seymore Butts. Not only is his mom proud of him, she works in his company’s office! There is an episode in the first season where she has to explain what she does for a living to a new neighbor, another middle-aged woman.
[/continued hijack]
I just want to point out what a sticky/icky phrasing that is.
Considering the difficulty of distinguishing friend vs. foe, especially given the wide variation in the haploid genotype of this kind of cell, and the potential disasters that can occur if the “killer sperm” scheme malfunctions, it’s probably far easier to just produce gobs and gobs of the stuff, or rely on more mechanical means of competition.
Many species have evolved specific penile anatomy designed to overcome previous suitors’ efforts, such as the goat, which delivers its payload with a “commando urethra” intended to insert the troops right at the cervix…
Or the canine, whose organ swells up for a time, ensuring that the boys have a period of noncompetion.
Or the horse and human, with a corona apparently designed to sweep out competitors’ swimmers before delivering its own. The horse also produces copious amounts of ejaculate, with the most potent delivered last… and ISTR that it “coagulates” into a plug.
There are cites for these anatomical facts on the web… but I can’t do that research very well here at the office. I think google searches for “goat urethra” would attract unwanted attention from the IT folks.
Maybe someone else more familiar with animal husbandry resources on the web can confirm my facts.
The ideas of “killer sperm,” sperm competition, and the influence of the female orgasm on sperm competition is extensively detailed in Robin Baker’s “Sperm Wars.”
As to the idea of multiple partners – my understanding is that the ideal of monogamy and the reality of infidelity have simultaneously always been part of the human condition and both have had enormous impacts on human evolution.
Heck, humans, horses, and dogs have nothing on some insects. The males of some species have tear-away, uh, equipment, and a gluey ‘payload’. This leaves a female glued up tight afterwards, keeping any other boys out.
It’s pretty much a one-and-done scenario for the males, but it seems to work.